function RenderManager() {

  this._renderCallbacks = [];

  this.registerRenderCallback = function(callback) {
    this._renderCallbacks.push(callback);
  }

  this.render = function() {

    var canvas = document.getElementById('graphCanvas');

    for (i = 0; i < this._renderCallbacks.length; i++) {
      this._renderCallbacks[i](canvas.getContext('2d'));
    }
  }
}